1. Field of the Invention
The present invention relates to a crankshaft of a type 6-cylinder combustion engine having four crank journals.
2. Description of Related Art
A crankshaft has an important function of producing dynamics by changing rectilinear motion generated by pistons of each cylinder into rotational motion, and, at the same time, applying force with the pistons that reach a bottom dead point to generate the rectilinear motion successively. The crankshaft should be able to rotate substantially without vibration, since the center of crank journals and crank pins are eccentric to each other, and the crankshaft cannot normally maintain a balance.
In order to solve such a problem, balance weights are generally provided to crank arms opposite to the crank pins to maintain balance during the rotation of the crankshaft.
While the balance weights used in a small-sized engine are formed integrally with the crank arms, those in a large-sized engine are made separately and attached to crank arms.
The balance weights are aligned in a symmetrical direction to offset inertia couple. Since the development angle of the crank arms in a series 4-internal combustion engine usually has 180 degrees, there is no problem in the disposition of the balance weights. In the case of the V-type 6-cylinder internal combustion engine, however, it is hard to position the balance weights. Because the weight of the crankshaft is excessively heavy, if the balance weights are provided to all the crank arms, torsional vibration occurs in the high speed region engine.
U.S. Pat. Nos. 4,552,104 and 4,730,512 disclose methods to solve such problems of the V-type 6-cylinder internal combustion engine.
Force of the crankshaft acting on both of its ends influences the size of inertia couple. In the above method, dynamic balance on the whole crankshaft is effectively achieved by disposing balance weights on both of its ends, and dynamic balance between adjacent main bearings is achieved by disposing balance weight on the midmost crank arm.
However, the balance weight provided to the midmost crank arm just influences the dynamic balance between the adjacent main bearings, and does not influence the dynamic balance on the whole crankshaft. Thus, the dynamic balance on the whole crankshaft depends on each pair of the balance weights disposed on both the ends of the crankshaft, and the size of the balance weights must be large. When it comes to molding of the crankshaft, a die has an upper-and-lower separating line. In the case of the balance weight disposed on the midmost crank arm, the separating line is on an adjacent crank pin, and upper and lower dies move in a direction of the center of gravity of the balance weight. Accordingly, it is too hard to mold a proper size of the balance weight.
A crankshaft disclosed in Japanese unexamined Publication No. Sho 60-227037 has six balance weights aligned in line to each crank arm to have 60 degrees of its disposition angle, and the inertia couple may be thereby relieved.
The third and fourth balance weights disposed on the crank arms adjacent to the midpoint of the crankshaft contribute to the dynamic balance between adjacent main bearings and the dynamic balance of the whole crankshaft, as well. Since the second and fifth balance weights are provided to the third and seventh crank arms, moment arm is thereby short, and this technique is not effective. In conclusion, the first and sixth balance weights should be formed large.